Jonathan Borden, stalwart of the RDDL effort, has created a new
experimental language, TDDL
(Terminology Definition Description Language), an XHTML based version of
RDF Schema.

Incorporating RDF Schema functionality, TDDL is designed to serve as
a way to define terminologies in a way that is both human and machine
readable. Borden writes about TDDL's creation:

This is an experiment, designed to highlight issues in describing
terminologies. I created TDDL during the process of developing RDDL simply
as an experiment and have not published these URLs. This topic came up for
discussion today on the RDFIG IRC channel and so I am publishing these URLs
for further discussion. TDDL was particularly inspired by DanC's HyperRDF.

For an example, see the TDDL definition
of TDDL itself. As Borden notes, his TDDL work is not part of the RDDL
specification, and is purely experimental.